The present invention relates to a disposable electro-analytical cell and a method and apparatus for quantitatively determining the presence of biologically important compounds such as glucose; hormones, therapeutic drugs and the like from body fluids.
Although the present invention has broad applications, for purposes of illustration of the invention specific emphasis will be placed upon its application in quantitatively determining the presence of a biologically important compoundxe2x80x94glucose.
Diabetes, and specifically diabetes mellitus, is a metabolic disease characterized by deficient insulin production by the pancreas which results in abnormal levels of blood glucose. Although this disease afflicts only approximately 4% of the population in the United States, it is the third leading cause of death following heart disease and cancer. With proper maintenance of the patient""s blood sugar through daily injection of insulin, and strict control of dietary intake, the prognosis for diabetics is excellent. The blood glucose levels must, however, be closely followed in the patient either by clinical laboratory analysis or by daily analyses which the patient can conduct using relatively simple, non-technical, methods.
At present, current technology for monitoring blood glucose is based upon visual or instrumental determination of color change produced by enzymatic reactions on a dry reagent pad on a small plastic strip. These colorimetric methods, which utilize the natural oxidant of glucose to gluconic acid, specifically oxygen, are based upon the reactions:
B-D-Glucose+O2+H2Oxe2x86x92D-Gluconic Acid+H2O2
H2O2+Reagentxe2x86x92H2O+color
Wherein glucose oxidase catalyzes the conversion of B-D Glucose to D-Gluconic Acid. The hydrogen peroxide produced is measured by reflectance spectroscopic methods by its reaction with various dyes, in the presence of the enzyme peroxidase, to produce a color that is monitored.
While relatively easy to use, these tests require consistent user technique in order to yield reproducible results. For example, these tests require the removal of blood from a reagent pad at specified and critical time intervals. After the time interval, excess blood must be removed by washing and blotting, or by blotting alone, since the color measurement is taken at the top surface of the reagent pad. Color development is either read immediately or after a specified time interval.
These steps are dependent upon good and consistent operating technique requiring strict attention to timing. Moreover, even utilizing good operating technique, calorimetric methods for determining glucose, for example, have been shown to have poor precision and accuracy, particularly in the hypoglycemic range. Furthermore, instruments used for the quantitative calorimetric measurement vary widely in their calibration methods: some provide no user calibration while others provide secondary standards.
Because of the general lack of precision and standardization of the various methods and apparatus presently available to test for biologically important compounds in body fluids, some physicians are hesitant to use such equipment for monitoring levels or dosage. They are particularly hesitant in recommending such methods for use by the patients themselves. Accordingly, it is desirable to have a method and apparatus which will permit not only physician but patient self-testing of such compounds with greater reliability.
The present invention addresses the concerns of the physician by providing enzymatic amperometry methods and apparatus for monitoring compounds within whole blood, serum, and other body fluids. Enzymatic amperometry provides several advantages for controlling or eliminating operator dependant techniques as well as providing a greater linear dynamic range. A system based on this type of method could address the concerns of the physician hesitant to recommend self-testing for his patients.
Enzymatic amperometry methods have been applied to the laboratory based measurement of a number of analytes including glucose, blood urea nitrogen, and lactate. Traditionally the electrodes in these systems consist of bulk metal wires, cylinders or disks imbedded in an insulating material. The fabrication process results in individualistic characteristics for each electrode necessitating calibration of each sensor. These electrodes are also too costly for disposable use, necessitating meticulous attention to electrode maintenance for continued reliable use. This maintenance is not likely to be performed properly by untrained personnel (such as patients); therefore, to be successful, an enzyme amperometry method intended for self-testing (or non-traditional site testing) must be based on a disposable sensor that can be produced in a manner that allows it to give reproducible output from sensor to sensor and at a cost well below that of traditional electrodes.
The present invention addresses these requirements by providing miniaturized disposable electroanalytic sample cells for precise micro-aliquot sampling, a self-contained, automatic means for measuring the electrochemical reduction of the sample, and a method for using the cell and apparatus according to the present invention.
The disposable cells according to the present invention are preferably laminated layers of metallized plastic and nonconducting material. The metallized layers provide the working and reference electrodes, the areas of which are reproducibly defined by the lamination process. An opening through these layers is designed to provide the sample-containing area or cell for the precise measurement of the sample. The insertion of the cell into the apparatus according to the present invention, automatically initiates the measurement cycle.
To better understand the process of measurement, a presently preferred embodiment of the invention is described which involves a two-step reaction sequence utilizing a chemical oxidation step using other oxidants than oxygen, and an electro-chemical reduction step suitable for quantifying the reaction product of the first step. One advantage to utilizing an oxidant other than dioxygen for the direct determination of an analyte is that such other oxidants may be prepositioned in the sensor in a large excess of the analyte and thus ensure that the oxidant is not the limiting reagent (with dioxygen, there is normally insufficient oxidant initially present in the sensor solution for a quantitative conversion of the analyte).
In the oxidation reaction, a sample containing glucose, for example, is converted to gluconic acid and a reduction product of the oxidant. This chemical oxidation reaction has been found to precede to completion in the presence of an enzyme, glucose oxidase, which is highly specific for the substrate B-D-glucose, and catalyzes oxidations with single and double electron acceptors. It has been found, however, that the oxidation process does not proceed beyond the formation of gluconic acid, thus making this reaction particularly suited for the electrochemical measurement of glucose.
In a presently preferred embodiment, oxidations with one electron acceptor using ferrocyanide, ferricinum, cobalt (III) orthophenanthroline, and cobalt (III) dipyridyl are preferred. Benzoquinone is a two electron acceptor which also provides excellent electro-oxidation characteristics for amperometric quantitation.
Amperometric determination of glucose, for example, in accordance with the present invention utilizes Cottrell current micro-chronoamperometry in which glucose plus an oxidized electron acceptor produces gluconic acid and a reduced acceptor. This determination involves a preceding chemical oxidation step catalyzed by a bi-substrate bi-product enzymatic mechanism as will become apparent throughout this specification.
In this method of quantification, the measurement of a diffusion controlled current at one or more accurately specified times (e.g., 5, 10, or 15 seconds) after the instant of application of a potential has the applicable equation for amperometry at a controlled potential (E=constant) of:       i          COTTRELL                        at          ⁢                      xe2x80x83                    ⁢          t                 greater than         0              =                    (                  nFA          ⁡                      (            Dt            )                          )                    -        0.5              ·          C              analyte                              at            ⁢                          xe2x80x83                        ⁢            t                    =          0                    
which may also be expressed as:       i    ⁡          (      t      )        =                              nFAC          metabolite                ⁡                  (          D          )                    0.5        ⁢                  (                  π          ⁢                      xe2x80x83                    ⁢          t                )                    -        0.5            
where i denotes current, nF is the number of coulombs per mole, A is the area of the electrode, D is the diffusion coefficient of the reduced form of the reagent, t is the preset time at which the current is measured, and C is the concentration of the metabolite. Measurements by the method according to the present invention of the current due to the reoxidation of the acceptors were found to be proportional to the glucose concentration in the sample.
The method and apparatus of the present invention permit, in preferred embodiments, direct measurements of blood glucose, cholesterol and the like. Furthermore, the sample cell according to the present invention, provides the testing of controlled volumes of blood without premeasuring. Insertion of the sample cell into the apparatus thus permits automatic functioning and timing of the reaction allowing for patient self-testing with a very high degree of precision and accuracy.
One of many of the presently preferred embodiments of the invention for use in measuring B-D glucose is described in detail to better understand the nature and scope of the invention. In particular, the method and apparatus according to this embodiment are designed to provide clinical self-monitoring of blood glucose levels by a diabetic patient. The sample cell of the invention is used to control the sampling volume and reaction media and acts as the electrochemical sensor. In this described embodiment, benzoquinone is used as the electron acceptor.
The basic chemical binary reaction utilized by the method according to one preferred embodiment of the present invention is:
B-D-glucose+Benzoquinone+H2Oxe2x86x92Gluconic Acidxe2x86x92Hydroquinone Hydroquinonexe2x86x92benzoquinonexe2x88x922e-+2H+.
The first reaction is an oxidation reaction which proceeds to completion in the presence of the enzyme glucose oxidase. Electrochemical oxidation takes place in the second part of the reaction and provides the means for quantifying the amount of hydroquinone produced in the oxidation reaction. This holds true whether catalytic oxidation is conducted with two-electron acceptors or one electron acceptor such as ferricyanide [wherein the redox couple would be Fe(CN)6xe2x88x923/Fe (CN)6xe2x88x924], ferricinium, cobalt III orthophenanthroline and cobalt (III) dipyridyl.
Catalytic oxidation by glucose oxidase is highly specific for B-D-glucose, but is nonselective as to the oxidant. It has now been discovered that the preferred oxidants described above have sufficiently positive potentials to convert substantially all of the B-D-glucose to gluconic acid. Furthermore, this system provides a means by which amounts as small as 1 mg of glucose (in the preferred embodiment) to 1000 mg of glucose can be measured per deciliter of samplexe2x80x94results which have not previously been obtained using other glucose self-testing systems.
The sensors containing the chemistry to perform the desired determination, constructed in accordance with the present invention, are used with a portable meter for self-testing systems. In use, the sensor is inserted into the meter, which turns the meter on and initiates a wait for the application of the sample. The meter recognizes sample application by the sudden charging current flow that occurs when the electrodes and the overlaying reagent layer are initially wetted by the sample fluid. Once the sample application is detected, the meter begins the reaction incubation step (the length of which is chemistry dependent) to allow the enzymatic reaction to reach completion. This period is on the order of 15 to 90 seconds for glucose, with incubation times of 20 to 45 seconds preferred. Following the incubation period, the instrument then imposes a known potential across the electrodes and measures the resulting diffusion limited (i.e., Cottrell) current at specific time points during the Cottrell current decay. Current measurements can be made in the range of 2 to 30 seconds following potential application with measurement times of 10 to 20 seconds preferred. These current values are then used to calculate the analyte concentration which is then displayed. The meter will then wait for either the user to remove the sensor or for a predetermined period before shutting itself down.
Due to the nature of the Cottrell current, it is possible to develop a calibration curve at more than one time point following application of the potential in order to verify that the measurement is being accurately made. Results can then be calculated at the different time points and compared. This is illustrated schematically in FIGS. 11 and 12; which indicate expected, or xe2x80x9cnormalxe2x80x9d Cottrell curves, A, B, C, D, for various glucose concentrations and an abnormal curve E, showing divergence from expected curve D as indicated by the multiple current readings. In a system that is operating correctly, the results should agree within reasonable limits. The exact range of acceptable difference between the expected and measured currents depends on a number of compromises but would generally be in the range of 1-10%. Results outside of the acceptable limits would indicate some problem with the system. For instance, incomplete wetting of the reagent (i.e., too small of a drop of blood) would result in failure to follow the Cottrell curve decay and result in a higher value being calculated at subsequent measurement points than would have been expected for Cottrell current curve delay. FIG. 13 represents a schematic circuit diagram which can be employed in producing a preferred embodiment of the invention for taking multiple current measurements.
The present invention provides for a measurement system that eliminates several of the critical operator dependant variables that adversely affect the accuracy and reliability and provides for a greater dynamic range than other self-testing systems.
These and other advantages of the present invention will become apparent from a perusal of the following detailed description of one embodiment presently preferred for measuring glucose and other analytes which is to be taken in conjunction with the accompanying drawings in which like numerals indicate like components and in which: